(a) Field of the Invention
The present invention relates to a projector-type head lamp for use on vehicles, and more particularly to a projector-type head lamp with which no colored light zone will appear near the light-dark limit of a luminous intensity distribution pattern formed by a light beam projected after shaped into an appropriate form by a shade disposed between a light source and a convex lens.
(b) Description of the Prior Art
Generally, the head lamps of a car are required to brightly illuminate the lane surface in front of the car in a luminous intensity distribution pattern which will not dazzle the driver of a car running on the opposite lane. To meet these requirements, the so-called projector-type head lamps have been proposed as a head lamp of which the optical system is simple and which can be designed compact as a whole. The optical system of one example of such head lamps of projector type is schematically shown in FIG. 1. Generally, the projector-type head lamp comprises a reflector 10 of which the inside reflective surface may be shaped in any of many different geometrical forms, for example, a spheroidal form. There is disposed an incandescent lamp 12 having a filament 14 positioned at the inner focus Fl of the reflector 10, and a shade 16 is disposed near the outer focus F2 to shape into an appropriate form the light reflected by the reflective surface. Also there is disposed a convex lens 18 having a focal plane i-j with which the outer focus F2 of the reflector 10 coincides. The light emitted from the light source is reflected by the reflector 10 and incident upon the outer focus F2 of the reflector 10 where it is shaped by the shade 16. The shade 16 has the top thereof cut as shown in FIG. 2 (cut line indicated with the reference number 20). The light incident upon the shade 16 is shaped as partially blocked, and the light thus shaped is projected frontward through the convex lens 18. The luminous intensity distribution pattern on a screen disposed in a position about 10 meters from the light source is shown in the form of an isolux curve in FIG. 4. With such a conventional protector-type head lamp, the light incident upon the convex lens 18 outgoes generally horizontally as considered from the standpoint of geometrical optics, but since the light source is a coiled tungsten filament, the light is not a light of single wavelength in practice. Hence, a phenomenon takes place that light beams different in wavelength from one another and incident upon the convex lens 18 are refracted in different directions depending upon their respective wavelengths. This is called a "dispersion". This light dispersion will be described with reference to FIG. 3. Among the light beams incident upon the upper portion of the convex lens 18, those of large wavelengths (lights going toward red) are refracted upward with respect to the horizontal direction, while the light beams of small wavelengths (lights going toward purple) are refracted downward with respect to the horizontal direction. Of the light beams incident upon the lower portion of the convex lens 18, those of large wavelengths are refracted downward with respect to the horizontal direction, while the light beams of small wavelengths are refracted upward with respect to the horizontal direction. With such a projector-type head lamp, as an influence due to the above-mentioned dispersion appears within a dark area 24 along a light-dark limit 22 defined by the cut line 20 of the shade 16, namely, since a colored light appears above the light-dark limit 22, an iridescent zone develops within the dark area 24. This phenomenon is caused by the light components refracted upwardly with respect to the horizontal direction, rather than by the light components refracted downwardly with respect to the horizontal direction. To reduce such dispersion, a so-called composite lens structure may be adopted, but this is not economic because its manufacturing costs are high. Further, in the conventional projector-type head lamps, the filament as light source is an axial or longitudinal coil which will cause an uneven brightness distribution in which zones of maximum and minimum brightness appear alternately. Also, the luminous intensity distribution pattern is influenced by such uneven brightness distribution.
In the case of a reflector in which the inside reflective surface is spheroidal and employed in a projector-type head lamp, the luminous intensity distribution pattern resulted from the converging of the light emitted from the filament and reflected by the reflective surface takes a peanut-like form, that is, the central upper and lower portions of the pattern are concave downward and upward, respectively. As shown in FIG. 4, the luminous intensity ,distribution pattern projected through the convex lens after being shaped by the shade also has the central lower portion thereof still remained concave. So, an improved optical system is needed to provide an ideal luminous intensity distribution pattern.